Typical automotive engines include a timing chain (hereinafter simply referred to as the “chain”) through which the rotation of the crankshaft is transmitted to the camshafts to rotate the camshafts, thereby opening and closing the valves of the combustion chambers. In order to keep the tension of the chain within an appropriate range, many of such engines further include a tension adjusting device comprising a chain guide pivotable about a pivot shaft, and a chain tensioner which presses the chain through the chain guide.
One known chain tensioner used in this tension adjusting device includes a tubular cylinder having an open end and a closed end, a tubular plunger having a bottom and an open end, and axially slidably inserted in the cylinder such that the open end is located in the cylinder, a return spring biasing the plunger in the direction in which the plunger protrudes out of the cylinder, an oil supply passage through which hydraulic oil is fed into a pressure chamber defined by the plunger and the cylinder, and a check valve provided at the end of the oil supply passage connected to the pressure chamber, and configured to allow only a flow of hydraulic oil from the oil supply passage toward the pressure chamber (JP Patent Publication 2009-79604A).
This chain tensioner is configured such that when the tension of the chain increases while the engine is running, the plunger is moved in the direction in which the plunger is pushed into the cylinder (this direction is hereinafter referred to as the “push-in direction”) under the tension of the chain, thus absorbing the tension of the chain. When the plunger is pushed in, hydraulic oil in the pressure chamber flows through a leak gap between the sliding surfaces of the plunger and the cylinder, thus damping and slowing down the movement of the plunger due to the viscous resistance of hydraulic oil.
When the tension of the chain decreases while the engine is running, the plunger is moved in the direction in which the plunger protrudes out of the cylinder (this direction is hereinafter referred to as the “protruding direction”) under the biasing force of the return spring, thereby eliminating slackness of the chain. When the plunger is pushed out, the check valve opens, causing hydraulic oil to flow from the oil supply passage into the pressure chamber, so that the plunger is pushed out quickly.
When the engine is stopped, since the oil pump also stops, the level of the hydraulic oil in the oil supply passage drops, so that a large amount of air remains in the oil supply passage. As a result, when the engine is restarted, a large amount of air tends to flow into the pressure chamber from the oil supply passage, and could get trapped in the plunger. If a large amount of air is trapped in the plunger when a load in the push-in direction is applied to the plunger, since the plunger moves while compressing the air trapped in the plunger, the damper performance of the chain tensioner deteriorates.
In order to prevent deterioration in the damper performance, this chain tensioner includes a mechanism for expelling air in the plunger.
This mechanism includes a cylindrical through hole extending through the plunger for communication between the interior and exterior of the plunger at its protruding end protruding out of the cylinder, a plug press-fitted in the cylindrical inner periphery of the through hole, and formed with a leak groove defined by an external thread on its outer periphery such that air in the plunger is discharged to the exterior of the plunger through the leak groove. By forming such an air vent passage, compared with an arrangement in which air vent passage is formed by press-fitting a cylindrical roller into a hole formed with an internal thread, it is possible to reduce burrs when the plug is press-fitted, and it is also possible to more easily remove burrs.
With the conventional chain tensioner described above, it is possible to change the hydraulic damper force by changing the setting of the leak gap. However, since the size of the leak gap changes with the difference in thermal behavior between the cylinder and the plunger, a change in temperature influences the hydraulic damper force generating characteristics. Namely, there is a large difference between the hydraulic damper force generating characteristics at normal temperature and at high temperature. In order to reduce the influence of a change in temperature, the setting of the leak gap is limited. Also, the hydraulic damper force generating characteristics required differ from one engine type to another because of the difference in engine behavior. Thus, it was basically necessary to design the internal structure of a chain tensioner for one engine type separately from a chain tensioner for another engine type.
An object of the present invention is to provide a chain tensioner configured such that the hydraulic damper force generated can be changed easily, and to provide chain tensioners which generate different hydraulic damper forces in an easy manner.